1. Technical Field
The present disclosure relates to an integrated magnetic sensor for detecting horizontal magnetic fields and manufacturing process thereof. In particular, the disclosure regards an integrated magnetic sensor provided with a magnetic concentrator.
2. Description of the Related Art
As is known, the integration of a thin ferromagnetic layer in a standard CMOS or CMOS-compatible process enables provision of integrated magnetic-field sensors, such as fluxgate sensors and Hall-effect sensors, capable of detecting horizontal magnetic fields.
An example of a fluxgate sensor of a planar type sensitive to external magnetic fields parallel to the surface of a chip is described in U.S. Pat. No. 6,404,192 and shown in FIG. 1. The sensor 1 comprises a magnetic core 4, here cross-like shaped, overlying an exciting coil 2, which in turn overlies four sensing coils 3a, 3b, 3c and 3d, arranged on a substrate (not shown). By supplying the exciting coil 2 with an appropriate a.c. excitation current, it is possible to bring each half-arm of the magnetic core 4 into a series of magnetic-saturation cycles that enable modulation and thus identification of the external magnetic field. Sensing of respective components of an external magnetic field that are parallel to the half-arms is obtained via the pairs of sensing coils 3a-3d. 
An example of a Hall-effect sensor 9 provided with a concentrator is shown in FIG. 2, wherein a substrate 10 of semiconductor material houses a pair of Hall cells 11. A concentrator 12 extends on the surface of the substrate 10, insulated therefrom via an insulating layer (not shown).
The concentrator 12 is formed by a strip of ferromagnetic material, the ends whereof are vertically aligned to a respective Hall cell 11, obtained in a known way and thus not shown in detail. Moreover this figure shows the lines of flux of an external magnetic field B having a distribution parallel to the surface of the substrate and deviated by the concentrator so as to traverse the Hall cells 11 in a direction perpendicular to the substrate surface and thus so as to enable sensing of the external field by the Hall cells, which are in themselves sensitive only to the components of the field perpendicular to the surface.
Using fluxgate technology it is possible to obtain sensors that are able to measure d.c. or slowly variable magnetic fields having an intensity of between a few μG and a few Gauss, with a high resolution, of the order of nG. In terms of dynamic range and resolution, fluxgate devices are positioned between Hall-effect magnetic-field sensors (which can typically detect fields of between 10 and 106 G) and other types of sensors—such as for example SQUID (Superconducting Quantum Interference Devices) sensors—which can detect fields of between 10−10 and 10−5 G.
For low values of magnetic field, fluxgate sensors are to be preferred to Hall-effect sensors on account of their better performance and find a wider application as compared to SQUID sensors, thanks to their lower cost and reduced overall dimensions.
To extend the sensitivity of fluxgate sensors, it has been already proposed to integrate Hall cells in a fluxgate sensor, to obtain a FluxHallGate sensor, as described in U.S. patent application Ser. No. 12/628,448, filed on Dec. 1, 2009, which is incorporated herein by reference in its entirety. In particular (see FIG. 3), a FluxHallGate sensor has the basic structure of the fluxgate sensor 1 of FIG. 1, and Hall cells 6 extend underneath the exciting coils 3a-3d. In FIG. 3, for reasons of simplicity, only two exciting coils 3a, 3b and the corresponding Hall cells 6a, 6b are shown, but the structure may also comprise a second pair of exciting coils (that are the same as the exciting coils 3c, 3d of FIG. 1), having respective Hall cells underneath.
In all these cases, the magnetic core 4 or the concentrator 12 enables an increase in the sensitivity of the sensor, thanks to the capacity of modifying the lines of flux of the magnetic field and concentrating them in proximity of the sensing coils 3a-3d and/or of the Hall-effect sensors 6.
However, current solutions may be improved. In fact, the effectiveness of the magnetic core or concentrator depends upon its distance from the sensitive element. On the other hand, providing the magnetic core or concentrator in proximity of the sensitive element is problematical. In fact, ferromagnetic materials generally contain iron, nickel, cobalt, and other contaminating elements that in some cases may lead to failure of the electronic components associated with the sensor. Consequently, the wafers are processed using dedicated apparatuses after depositing the ferromagnetic material. The greater the number of technological steps executed after deposition of the ferromagnetic material, the greater the number of apparatuses that are used only to process the wafers with magnetic sensors and the higher the costs.
The problems referred to above are all the more serious when the associated read circuit is integrated with the sensor: for example, it would be necessary to have particularly thick oxide layers causing the manufacture of deep vias to be particularly complex on account of the high aspect ratio. In addition, since the ferromagnetic layer generally has a wide area, problems may arise also in positioning the vias. For these reasons, it would be desirable to be able to form the ferromagnetic layer at the end of the manufacturing process, immediately prior to passivation and opening of the pads, but this technological requirement conflicts with the obtaining a high sensitivity.